The present invention relates to a ceramic varistor having a capability of storing a great amount of energy.
Voltage-dependent resistors (varistors) having a non-linear voltage-current characteristic are adapted to inhibit an overvoltage encountered by electric equipment.
Recently, ceramic varistors mainly consisting of zinc oxide have been developed and applied in various fields because of their excellent performance. When their applications are further extended, a greater energy storing capability (to be referred to as "withstand energy") is desired. Furthermore, when the applications to general electronic equipment are taken into consideration, it is desired that performance may be maintained while the size is reduced.
The construction of a ceramic varistor is such that a pair of electrodes are formed on opposite surfaces of a sintered body mainly consisting of zinc oxide. Its microstructure is such that boundary layers consisting of additives surround particles of zinc oxide and they are connected in rows and columns. Zinc oxide particles G have a resistivity from 1 to 10 ohm-cm while the boundary layers, a resistivity higher than 10.sup.10 ohm-cm. Therefore, when an overvoltage is applied to the electrodes, almost all of the charges are applied to the boundary layers where they are subjected to the thermal conversion and consumed, whereby the equipment or the like may be protected. A great factor which determines the withstand energy of the ceramic varistor is a thermal capacity of zinc oxide particles. The improvement of the withstand energy is possible by the increase in size of zinc oxide particles. However, because the ceramic techniques are used for the production of the ceramic varistors and because of the effects of the additives and other characteristic items, the expectation for the growth of zinc oxide particles is limited. Furthermore, to this end special means (production steps), are required.